IRS-8 - A unique bow shock in the Galactic Center region

November 6, 2002

Perhaps flung from the Galactic Center's innermost cluster of hot stars some 20,000 years ago, the massive star IRS-8 is moving at supersonic speed through high density gas.

Located
8000 parsecs from the Sun in the direction of Sagittarius, the nucleus
of the Milky Way is host to numerous high energy phenomena including a
supermassive black hole of approximately three million solar masses
which defines the dynamic center of our galaxy. The nucleus also
contains a number of young clusters of recently formed hot and massive
stars, peculiar radio-emitting filaments of ionized gas, supernova
remnants, and a number of bright and compact objects whose natures and
origins are unclear.

Because of the high dust content
along the line of sight to the Galactic Center, it has been impossible
to observe this region at ultraviolet and optical wavelengths. New
tools such as infrared array detectors combined with adaptive optics on
large 8-10 m telescopes have given astronomers new ways to explore this
unique region of the nearby universe. For example, see the work
published recently by Schödel et al. in Nature, Volume 419 October 17, 2002 pages 694-696.

Tom
Geballe, Francois Rigaut and Jean-Rene Roy of the Gemini Observatory
have analyzed the exquisite images obtained with the University of
Hawaii's adaptive optics camera Hokupa'a-QUIRC on the Gemini North
Telescope in order to study the unique object IRS-8, which is located
30 arcsec (= 1.2pc) north of the Galactic Center (Figure 1). This
object has been known to be a strong infrared source for almost two
decades, but the Hokupa'a infrared images (H and K') have revealed that
the short wavelength IR emission from IRS-8 is largely in the form of a
spectacular bow shock (Figure 2) surrounding a central star. The dust
in the bow shock is heated and radiates because it is compressed by the
shock and because it absorbs ultraviolet radiation from hot stars
nearby, possibly including IRS-8's central star itself.

Interstellar
bow shocks are produced when massive stars with envelopes or winds move
at supersonic speeds (e.g. several tens or few hundreds km/sec) into
the surrounding gas/dust of the interstellar medium. The stellar wind
acts as a high pressure gas bubble surrounding the star. The bow shock
is created at the interface where the force due to the kinematic push
(ram pressure) of the moving (supersonic) stellar wind bubble equalizes
the thermal pressure of the local interstellar gas into which the star
is moving at high speed. The phenomenon is not unlike the bows created
by ships or other crafts moving in water (except the water does not
compress). With its apex at 0.01 pc (1000 AU) from the central star,
the IRS-8 bow shock is a cosmic version of this phenomenon. Still, this
stand-off distance is about 100 times smaller than all other known bow
shocks due to stars speeding through interstellar space.

One
reason for this compactness is the high density of the gas and dust
into which IRS-8 is moving. Independent infrared and radio observations
indicate that the density of the gas/dust is 100 to 1000 times higher
than the average interstellar density in the Milky Way. IRS-8 may
actually be colliding with the "northern arm", a region of high density
molecular gas that has been mapped by infrared and radio astronomers.
The arm seems to intersect the east side of IRS-8 bow shock, possibly
explaining why this side of the bow is brighter than the west side.

The
second reason for the bow shock's tightness is the high velocity of the
star. If IRS-8's stellar wind properties are normal, the bow shock
geometry and size require a space velocity of 150 km/s. Such a space
velocity is unusually high for a young massive star far from the
Galactic center, but is not unusual for objects within 1-2pc of the
supermassive black hole. The axis of the bow shock gives the direction
of the motion; IRS-8 appears to be moving to the northeast. It is also
possible that IRS-8 was flung out of one the young clusters of stars,
perhaps the central cluster located to its south about 20,000 years
ago. The mechanisms known to be able to eject stars are either the
explosion of one component of a binary massive system, or ejection from
the cluster by multiple star interactions. In both cases, the space
velocities acquired by the ejected stars can be as high as a few
hundreds km/sec.

The study of objects like IRS-8
helps us to understand both the evolution of massive stars and clusters
and the properties of the interstellar gas in the extreme environment
of the Galactic center.

Geballe, Rigaut and Roy will present their results at the Galactic Center Workshop, held in Kona, November 2002.

The Gemini Observatory is an international collaboration with two identical 8-meter telescopes. The Frederick C. Gillett Gemini Telescope is located on Maunakea, Hawai'i (Gemini North) and the other telescope on Cerro Pachón in central Chile (Gemini South); together the twin telescopes provide full coverage over both hemispheres of the sky. The telescopes incorporate technologies that allow large, relatively thin mirrors, under active control, to collect and focus both visible and infrared radiation from space.

The Gemini Observatory provides the astronomical communities in five partner countries with state-of-the-art astronomical facilities that allocate observing time in proportion to each country's contribution. In addition to financial support, each country also contributes significant scientific and technical resources. The national research agencies that form the Gemini partnership include: the US National Science Foundation (NSF), the Canadian National Research Council (NRC), the Argentinean Ministerio de Ciencia, Tecnología e Innovación Productiva, the Brazilian Ministério da Ciência, Tecnologia e Inovação and the Chilean Comisión Nacional de Investigación Científica y Tecnológica (CONICYT). The observatory is managed by the Association of Universities for Research in Astronomy, Inc. (AURA) under a cooperative agreement with the NSF. The NSF also serves as the executive agency for the international partnership.